Check DNS for both forward and reverse lookup. I had a similar error where C couldn't do a successful reverse lookup against internal/public DNS for E. Actually, internal DNS had a PTR, but public was missing it.
Check SRV records on public DNS (you...
Also,
1. Validate PTR for E is configured on public and private DNS.
2. Internal DNS should resolve External IP for E's FQDN.
3. C needs to be able to resolve E's IP to FQDN and FQDN back to IP. You can use DNS utility in both C and E to perform thes...
